Due to the technical limitations of the power quality (PQ) management of the chemical industry, metallurgy and other energy-intensive enterprises with non-linear power loads, there are a series of PQ issues to public grid, such as harmonic pollution, low power factor and high operating loss.
In order to solve the PQ issues of industrial rectifier system, the main measures are passive power filtering (PPF) method and active power filtering (APF) method. The PPF is generally considered as an economic choice for PQ improvement because of its low investment cost. But it can only filter out the specific order harmonics, and the filtering performance will be seriously affected when the harmonic source changes dynamically. By adopting the APF method, it can realize the real-time tracking and the suppression of harmonic currents. However, the large-capacity APF has shortcomings of high investment cost, much maintenance workload and the like, and is unlikely to be popularized for use in a large scale.
In the recent years, an inductive power filtering (IPF) method was proposed for industrial DC power supply system. The IPF method not only suppresses the harmonic currents but also prevents harmonic current from flowing into the primary winding of the rectifier transformer, which means that it can reduce the effects of harmonic components on the transformer. However, it is worth noting that the PPF-based IPF method still has the risk of resonance. Especially for the industrial distribution network which has a high content of harmonic background voltage, the IPF method has a weak ability for damping the parallel/series harmonic resonance. Besides, the IPF method requires the dual zero impedance design for the filtering winding and filtering branches. During the practical transformer design, by adjusting the values of the short-circuit impedances, the value of the equivalent impedance of the filtering winding may, ideally, be approximately equal to 0. As for the single tuned filter, due to the limitation of the manufacture technics, cost control and the existence of the transmission line impedance, while the capacitor and the inductor have been well tuned at the considered harmonic frequency in the process of design and manufacture, the equivalent impedance Zfn of the filtering branches still exists as a nonzero resistance. It can be known after the investigation of the reactor manufacturers that, the cost price will increase tenfold as the resistance inherent in the filtering branches reduces tenfold. And the coil wire with larger cross-sectional area is needed for a inductor with a low internal resistance. Therefore, to satisfy the design value of the quality factor (Q) of the filtering branches, the fabricating cost of the filtering device gets higher, and the installed space is greatened as well. For the energy-intensive enterprises which have more considerations for the cost of investment, it causes difficulty on the promotion and realization of the IPF method.